Multiple inverter power control systems in an energy generation system

ABSTRACT

A power control system includes a first inverter power control system and a second inverter power control system coupled in a parallel configuration with the first inverter power control system. Both first and second inverter power control systems may each include an input configured to receive direct current (DC) power; a DC to alternating current (AC) inverter stage configured to receive the DC power input; an anti-islanding relay coupled to the output of the DC/AC inverter stage; and a transition relay coupled to the anti-islanding relay. The transition relay may be configured to route an output of the inverter power control system between one or more onsite back-up loads and an AC grid. The first inverter power control system may be designated as a master that is configured to control the operation of the second inverter power control system designated as a slave.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/205,452, filed on Aug. 14, 2015, which is herein incorporated byreference in its entirety for all purposes.

BACKGROUND

Decreasing costs, state and federal tax incentives, and increasedawareness of the correlation between greenhouse gasses (e.g., carbondioxide emissions) and climate change have increased the popularity ofphotovoltaic (PV) or “solar” energy generation systems with consumers,businesses and utility companies. A conventional solar energy generationsystem includes an array of PV modules connected together on one or morestrings, a combiner for combining direct current (DC) outputs of the oneor more strings, one or more string inverters for converting thecombined DC output from the strings to alternating current (AC), and aphysical interface to AC grid power—typically on the load side of theutility meter, between the meter and the customer's main electricalpanel. Alternatively, micro inverters may be used with each panel or Npanels (N<=4), obviating the need for a string inverter. Theconventional solar energy generation system provides excess AC powerback to the AC grid, resulting in cost benefits to the customer.

Conventional solar energy generation systems have been improved toinclude on-site energy storage. Including energy storage in conventionalsolar energy generation systems improves the functionality andversatility of such systems. For instance, including on-site energystorage provides a potential source of power when the grid isunavailable, such as when an outage occurs. Additionally, it allows thecustomer to store the energy generated during the day when the solararray is generating the most power, and then consume that generatedpower after the sun has set, thereby reducing the customer's peakdemand. Distributed storage even has the potential to benefittraditional utilities by allowing their customers to supply power backto the grid at a time when the grid needs additional power, such as tomeet mid-day load when active heating or cooling are in use across thegrid. Localized energy storage can help utilities stabilize the grid bysupplying power to enhance demand response, shaving demand peaks, andshifting loads to times of lower demand. In fact, storage could eventake power from the grid instead of the PV system during the middle ofthe night when supply is high and demand is low. Furthermore, byenabling customers to store energy onsite, it may be possible to billcustomers for energy supplied to back-up loads when the grid isunavailable (e.g., during an outage). Managing the operation of suchenergy generation systems can be very complex. Power can flow betweenseveral components in the energy generation system, and successfuloperation of the energy generation system relies on the absence ofconflicting power flow. Given the benefits of solar energy generationsystems having on-site energy storage, improvements to the management ofpower flow in such systems are desired.

SUMMARY

Embodiments describe solar energy generation systems that increase thefunctionality and versatility of the solar energy generation systemshaving on-site energy storage. In some embodiments, energy generationsystems may have a power control system containing more than oneinverter power control system (PCS). The inverter PCSs may not onlycontrol the transfer of power between DC sources (e.g., PV array andenergy storage devices) and an AC grid and/or back-up loads, but alsocontrol the transfer of power between each inverter PCS. One of theinverter PCSs may be configured to manage the operation of the otherinverter PCS.

In other embodiments, energy generation systems may have an energystorage system containing a plurality of energy storage devices. Theinverter PCS may store energy into, and withdraw energy from, theplurality of energy storage devices within the energy storage system.The energy storage systems may be configured to directly receivecommands from the inverter PCS for operating the energy generationsystem. One of the energy storage devices may be configured to managethe operation of the other energy storage devices within the energystorage system. It is to be appreciated that an energy storage systemaccording to disclosures herein do not include an inverter PCS, but mayinclude one or more energy storage devices and one or more othercomponents.

In yet other embodiments, energy generation systems may have severalpower subsystems that provide power to different phases of an AC grid.Each power subsystem may include an energy generation device, inverterPCS, and energy storage device that provides power to an AC grid or aback-up load, both of which operate in each phase of the AC power. Oneof the inverter PCSs of a power subsystem may be configured to managethe operation of other inverter PCSs in other power subsystems of theenergy generation system.

In an embodiment, a power control system includes a first inverter powercontrol system and a second inverter power control system coupled in aparallel configuration with the first inverter power control system.Both first and second inverter power control systems may each include aninput configured to receive direct current (DC) power; a DC toalternating current (AC) inverter stage configured to receive the DCpower input; an anti-islanding relay coupled to the output of the DC/ACinverter stage; and a transition relay coupled to the anti-islandingrelay. The transition relay may be configured to route an output of theinverter power control system between one or more onsite back-up loadsand an AC grid. The first inverter power control system may bedesignated as a master that is configured to control the operation ofthe second inverter power control system designated as a slave.

The first inverter power control system may be configured to control thetransition relay of the second inverter power control system. In someembodiments, both of the first and second inverter power control systemsfurther include a DC to DC converter stage configured to receive andstep up or step down the DC power input to a level suitable forinversion. The power control system may further include a communicationline coupled between the first and second inverter power controlsystems. In embodiments, the first and second inverter power controlsystems include a first and second communication device, respectively,the communication line coupling the first communication device to thesecond communication device. The communication line may be a wirelesscommunication line.

In certain embodiments, an energy generation system may include one ormore photovoltaic (PV) strings and a power control system including aplurality of inverter power control systems connected in a parallelconfiguration and coupled to the one or more PV strings. Each inverterpower control system may include an input configured to receive directcurrent (DC) power; a DC to alternating current (AC) inverter stageconfigured to receive the DC power input; an anti-islanding relaycoupled to the output of the DC/AC inverter stage; a transition relaycoupled to the anti-islanding relay, where the transition relayconfigured to route an output of the inverter power control systembetween one or more onsite back-up loads and an AC grid, and where oneof the plurality of inverter power control systems is designated as amaster that is configured to control an operation of another inverterpower control system designated as a slave; and one or more energystorage devices coupled to the plurality of inverter power controlsystems.

The energy generation system may also include a central AC disconnectcoupled between the plurality of inverter power control systems and theAC grid. The central AC disconnect may be configured to simultaneouslyconnect and disconnect the plurality of inverter power control systemsto the AC grid. In embodiments, the energy generation system may alsoinclude a communication line coupling the central AC disconnect with theplurality of inverter power control systems. In some embodiments, oneinverter power control system may be configured to control the otherinverter power control systems of the plurality of inverter powercontrol systems. Each inverter power control system may further includea DC to DC converter stage configured to receive and step up or stepdown the DC power input to a level suitable for inversion. In certainembodiments, the energy generation system may also include communicationlines coupled between the plurality of inverter power control systems.Each inverter power control system of the plurality of inverter powercontrol systems may include a communication device, where thecommunication lines couple together the communication devices in theplurality of inverter power control systems. The communication lines maybe wireless communication lines.

In some embodiments, a method includes receiving direct current (DC)power at a first inverter power control system and at a second inverterpower control system; generating a command at the first inverter powercontrol system; sending the command from the first inverter powercontrol system to the second inverter power control system; andreceiving, at the second inverter power control system, the command fromthe first inverter power control system, the command instructing thesecond inverter power control system to output power to at least one ofa plurality of destinations.

At least a portion of the received DC power at the second inverter powercontrol system may be outputted to an energy storage device. In someembodiments, the command may instruct the second inverter power controlsystem to alter a position of a transition relay in the second inverterpower control system. The position of the transition relay may bealtered to output power to an AC grid. The position of the transitionrelay may be altered to output power to on-site back-up loads.

In embodiments, an energy generation system may include an inverterpower control system and a plurality of energy storage devices coupledto the inverter power control system, where each energy storage deviceconfigured to communicate with the inverter power control system. Theinverter power control system may include an input configured to receiveDC power; a DC/AC inverter stage configured to receive the DC powerinput; an anti-islanding relay coupled to the output of the DC/ACinverter stage; and a transition relay coupled to the anti-islandingrelay, where the transition relay configured to route an output of theinverter power control system between one or more onsite back-up loadsand an AC grid.

The energy generation system may further include a battery combiner box,where the battery combiner box is coupled between the plurality ofenergy storage devices and the inverter power control system. Thebattery combiner box may be configured to combine DC power outputted bythe plurality of energy storage devices into a DC power bus. The batterycombiner box may include one or more disconnection and protectioncomponents configured to sever a flow of power between an energy storagedevice and the inverter power control system. Each of the plurality ofenergy storage devices may include a DC/DC converter to step up or stepdown a voltage of the output of the inverter power control system. Insome embodiments, the inverter power control system may becommunicatively coupled to the plurality of energy storage devicesthrough communication lines arranged in a parallel configuration. Thecommunication lines may be wireless communication lines or power linesthough which power is transferred. The inverter power control system maycommunicate with the plurality of energy storage devices through powerline communication.

In certain embodiments, an energy generation system includes an inverterpower control system configured to route power between an AC grid andone or more back-up loads; a plurality of PV panels to input DC power tothe inverter power control system; and a plurality of energy storagedevices coupled to the inverter power control system. The plurality ofenergy storage devices may include a master energy storage deviceincluding a buck-boost circuit and a communication circuit forcommunicating with the inverter power control system; and at least oneslave energy storage device coupled to the master energy storage device,where the at least one slave energy storage device is controlled by themaster energy storage device.

The at least one slave energy storage device may be coupled to themaster energy device through power lines arranged in a parallelconfiguration. In embodiments, the at least one slave energy storagedevice may be coupled to the master energy device through power linesarranged in a serial configuration. Each slave energy storage device mayhave less components than the master energy storage device. The inverterpower control system may be communicatively coupled to the master energystorage device through a communication line.

In certain embodiments, a method includes receiving direct current (DC)power at an inverter power control system; generating commands by acontroller in the inverter power control system; sending the commandsfrom the controller to a plurality of energy generation systems throughone or more communication lines; and receiving, by the plurality ofenergy storage devices, the commands from the inverter power controlsystem, the commands instructing the plurality of energy storage devicesto charge or discharge according to a charge or discharge scheme. Thecommands may be sent simultaneously to each energy storage device of theplurality of energy storage devices through the one or morecommunication lines arranged in a parallel configuration. In someembodiments, the commands may be sent in sequential order to each energystorage device of the plurality of energy storage devices through theone or more communication lines arranged in a serial configuration. Thecharge or discharge scheme may charge or discharge one energy storagedevice at a time and in a sequential order. In some embodiments, atransition of power between sequential energy storage devices mayinclude gradually decreasing a power output of one energy storage devicewhile gradually increasing a power output of another energy storagedevice. In certain embodiments, a transition of power between sequentialenergy storage devices may include equally charging or discharging eachenergy storage device of the plurality of energy storage devices at asame rate.

In embodiments, an energy generation system includes a plurality ofenergy generation devices for generating DC power; a plurality of energystorage devices for storing the generated DC power and dischargingstored DC power; and a plurality of single-phase inverters coupled torespective energy generation devices and energy storage devices, whereeach single-phase inverter of the plurality of single-phase inverters isconfigured to convert generated DC power or stored DC power to AC powerso that the converted AC power of each single-phase inverter is offsetby a phase from one another.

One of the plurality of single-phase inverters may be designated as amaster and the other single-phase inverters are designated as slaves.The master single-phase inverter may be configured to manage theoperation of the slave single-phase inverters. In some embodiments, theplurality of single-phase inverters may be communicatively coupled toone another by communication lines. The communication lines may bewireless communication lines or power lines though which power may betransferred. A frequency and voltage amplitude of each converted ACpower from the plurality of single-phase inverters may be equal to oneanother. The phase may be a third of a period of a waveform of the ACpower. In some embodiments, each single-phase inverter may be configuredto output the converted AC power to a respective AC grid or a respectiveback-up load. The respective AC grid may operate at the same frequency,voltage amplitude, and phase as the corresponding single-phase inverter.

In some embodiments, an energy generation system may include a firstsubsystem including a first inverter power control system (PCS)configured to output alternating AC power in a first phase that may beconverted from at least one DC power source; a second subsystemincluding a second inverter PCS configured to output AC power in asecond phase that may be converted from the at least one DC powersource; and a third subsystem including a third inverter PCS configuredto output AC power in a third phase that may be converted from the atleast one DC power source, where the first, second, and third phases maybe equally offset in phase from one another.

The first inverter PCS may be designated as a master and the second andthird inverter PCSs may be designated as slaves. The first inverter PCSmay manage operations of the second and third inverters to establish theoffset from one another. In embodiments, The first, second, and thirdinverter PCSs may be communicatively coupled to one another bycommunication lines.

In some embodiments, a method includes receiving direct current (DC)power at a first single-phase inverter power control system (PCS),second single-phase inverter PCS, and third single-inverter PCS;generating one or more commands at the first single-phase inverter PCS;sending the one or more commands from the first single-phase inverterPCS to the second and third single-phase inverter PCSs; and receiving,at the second and third single-phase inverter PCSs, the one or morecommands from the first single-phase inverter PCS. The one or morecommands may instruct the second single-phase inverter PCS to outputpower to at least one of a plurality of destinations in a second phase,and instruct the third single-phase inverter PCS to output power to atleast one of a plurality of destinations in a third phase. The firstsingle-phase inverter PCS may output power to at least one of aplurality of destinations in a first phase.

The one or more commands may instruct the first, second, and thirdsingle-phase inverter PCSs to output power in a first, second, and thirdphase that may be equally offset from one another. In some embodiments,the one or more commands instructs the first, second, and thirdsingle-phase inverter PCSs to output power in a first, second, and thirdphase offset by 120°. The first single-phase inverter PCS may bedesignated as a master and the second and third single-phase inverterPCSs may be designated as slaves. The one or more commands may be sentthrough communication lines coupled between the first single-phaseinverter PCS and the second and third single-phase inverter PCSs. Thecommunication lines may be arranged in a parallel or serialconfiguration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified block diagram of an energy generation systemincluding one array of PV strings, inverter PCS, and one energy storagedevice for outputting to AC grid/back-up loads.

FIG. 1B is a simplified block diagram of an energy generation systemincluding more than one array of PV strings and more than one energystorage device, according to embodiments of the present invention.

FIG. 2 is a simplified block diagram of an energy generation systemincluding a power control system having two inverter power controlsystems, according to embodiments of the present invention.

FIG. 3 is a simplified block diagram illustrating details of a powercontrol system for an energy generation system, according to embodimentsof the present invention.

FIG. 4 is a simplified block diagram illustrating details of a centralAC disconnect for an energy generation system, according to embodimentsof the present invention.

FIGS. 5A and 5B are simplified block diagrams illustrating a method ofoperating a central AC disconnect to prevent damage to an energygeneration system, according to embodiments of the present invention.

FIG. 6 is a simplified block diagram illustrating a master inverter PCSconfigured to store energy in energy storage devices coupled to itselfand slave inverter PCSs, according to embodiments of the presentinvention.

FIG. 7 is a simplified block diagram illustrating an energy generationsystem including one inverter PCS and a plurality of energy storagedevices, according to embodiments of the present invention.

FIG. 8A is a simplified block diagram illustrating details of an energygeneration system where an inverter PCS is communicatively coupled toeach energy storage device in a parallel (independent) configuration,according to an embodiment of the present invention.

FIG. 8B is a simplified block diagram illustrating details of an energygeneration system where an inverter PCS is communicatively coupled toeach energy storage device in a serial (i.e., daisy-chain)configuration, according to an embodiment of the present invention.

FIG. 9 is a simplified block diagram illustrating details of energystorage devices and a battery combiner box for an energy generationsystem where the battery combiner box has protection components,according to an embodiment of the present invention.

FIG. 10 is a series of charts illustrating various power sharing schemesamong multiple energy storage devices during charging/discharging,according to embodiments of the present invention.

FIG. 11 is a simplified block diagram illustrating details of multipleenergy storage devices and a battery combiner box for an energygeneration system where the battery combiner box has a multi-poletransfer switch and a single protection component, according to anembodiment of the present invention.

FIG. 12 is a simplified block diagram illustrating a simplified energystorage device having a plurality of energy storage devices connected ina serial configuration, according to one embodiment of the presentinvention.

FIG. 13 is a simplified block diagram illustrating a simplified energystorage device having a plurality of energy storage devices connected ina parallel configuration, according to one embodiment of the presentinvention.

FIG. 14A is a graph of a waveform for single-phase AC power.

FIG. 14B is a graph of waveforms for three-phase AC power.

FIG. 15A is a simplified diagram illustrating a circuit implementing a“Wye” configuration for a three-phase energy generation system.

FIG. 15B is a simplified diagram illustrating a circuit implementing a“Delta” configuration for a three-phase energy generation system.

FIG. 16 is a simplified diagram illustrating a three-phase energygeneration system, according to embodiments of the present invention.

DETAILED DESCRIPTION

Recently, inverter power control systems have been developed forcharging and discharging power with storage devices that are coupled toon-site power generation systems, on-site loads, and the grid. Thesesystems allow multiple modes of operation whereby a PV solar array cancharge the energy storage device or it can supply the harvested energyto an AC grid and/or on-site back-up loads. Alternatively, the energystorage device can supply energy to the AC grid and/or on-site back-uploads, either alone or combined with the solar array as applicable. Inanother alternative mode, the AC grid can be used to charge the energystorage device. Such systems are described in related U.S. patentapplication Ser. No. 14/798,069, filed Jul. 13, 2015, as well as U.S.Provisional Patent Application No. 62/151,257, filed Apr. 22, 2015,which are herein incorporated by reference in their entirety for allpurposes.

In embodiments, an energy generation system may be specially configuredto satisfy demands of an installation site. In a first example, anenergy generation system may be configured to output a greater magnitudeof power to an AC grid or back-up loads that require a greater amount ofpower to operate, such as for a large installation site that has manyloads (e.g., a villa, commercial building, multi-storied house and thelike). In a second example, an energy generation system may beconfigured to output a defined amount of power for a longer period oftime at an installation site that demands an extended energy capacitysuch as for an installation location at a remote site that does notreceive consistent amounts of sunlight. In a third example, an energygeneration system may be configured to output power to different phasesof an AC grid independently for an installation site that has back-uploads that run on different phases of power. Each of these examples willbe explored in more detail further herein.

I. Energy Generation System Configured for Large Power Consumption

In order to provide a greater amount of energy storage capacity and/orgreater power, it may be desirable to utilize a plurality of inverterPCSs and/or energy storage devices. For example, according toembodiments, a solar energy generation system may include at least twoinverter PCSs with an energy storage device or at least one inverter PCSwith two energy storage devices. Each inverter PCS may be configured toreceive DC power generated by an energy generation device, such as anarray of PV strings. The received DC power may then be either stored inan energy storage device (e.g., battery) or converted and outputted asAC power to an AC grid or a back-up load. According to embodiments, oneinverter PCS may be designated as a master, while other inverter PCSsmay be designated as slaves. The master inverter PCS may manage theoperation of slave inverter PCSs to increase the functionality andversatility of the solar energy generation system as a whole.

FIG. 1A illustrates a block diagram of an exemplary solar energygeneration system 100. One or more energy generation devices, e.g.,array of PV strings 102, may generate and output DC power to a powercontrol system 106. Power control system 106 may be coupled to an energystorage device 104 for storing DC energy, and to AC grid 108 and back-uploads 110 for outputting AC power. Energy storage device 104 may be anysuitable device capable of storing energy, such as a battery, fuel celland the like. AC grid 108 may be a utility grid or any other electricalcomponent for interfacing with the utility grid, such as a main panel,transformer, load center, and/or a substation. Back-up loads 110 may beany electrical device that utilizes power, e.g., a residential applianceor a commercial tool. Power control system 106 may be a three-phase orsingle-phase power control system that outputs power to a correspondingthree-phase or single-phase AC grid 108 and/or corresponding three-phaseor single-phase back-up loads 110. The operational phase of back-uploads 110 may depend on the operational phase of power control system106.

In embodiments, an inverter power control system 106 is configured tomanage the flow of power between the different components of solarenergy generation system 100. As an example, power control system 106may store DC power outputted from PV strings 102 into energy storagedevice 104, which may then be discharged at an opportune time to provideDC power that is converted into AC power and outputted into AC grid 108or back-up loads 110. In other examples, power control system 106 mayconvert DC power generated by PV strings 102 into AC power and thenoutput that AC power directly to AC grid 108 or back-up loads 110. Inyet other examples, power control system 106 may convert a portion ofthe total DC power from PV strings 102 into AC power for output to ACgrid 108 or back-up loads 110, and store the remaining DC power toenergy storage device 104.

Managing the flow of power between the components of energy generationsystem 100 includes managing power flow between one array of PV strings102, one energy storage device 104 and AC outputs (e.g., AC grid 108 andback-up loads 110). In embodiments where more than one array of PVstrings and more than one energy storage device are incorporated in theenergy generation system, managing the flow of power between thesedifferent components may be more complex.

FIG. 1B illustrates an exemplary energy generation system 101 havingmore than one array of PV strings: PV strings 112A and 112B, and morethan one energy storage device: energy storage device 114A and 114B. Tomanage the flow of power in energy generation system 101, power controlsystem 116 needs to route power between more PV strings and energystorage devices than power control system 106 in energy generationsystem 100 shown in FIG. 1A. Larger numbers of PV strings and energystorage devices greatly increases the complexity and logistics ofmanaging power flow across the energy generation system. Inefficientpower flow management may result in under-utilization of the energygeneration system and decreased performance, ultimately resulting inmonetary losses to the customer.

According to embodiments of the present invention, power control system114 may include more than one inverter power control system (PCS). Theinverter PCSs may work in concert to manage the flow of power across thevarious components in energy generation system 101. One inverter PCS maybe designated as a master inverter PCS and configured to manage theoperations of the other inverter PCSs designated as slave inverter PCSs,as will be discussed in detail further herein with respect to FIG. 2.

A. Inverter Power Control System

FIG. 2 illustrates an exemplary energy generation system 200 accordingto embodiments of the present invention. As shown, two arrays of PVstrings 202A and 202B are coupled to inputs of respective inverter PCSs206A and 206B. Each array of PV strings may include a plurality of PVmodules (not shown) connected serially and/or in parallel with anadditive direct current (DC) voltage somewhere between 100 and 1000volts, depending on such factors as the number of panels, theirefficiency, their output rating, ambient temperature and irradiation oneach panel. Also, each array of PV strings may include a maximumpower-point tracking (MPPT) system for maximizing the power output ofeach array of PV strings under different voltage conditions. In someembodiments, each MPPT system may receive the output of one or moreseparate strings connected in parallel (i.e., a two (or more)-to-onecombiner at each MPPT channel input), thus resulting in a dual MPPTsystem as shown in FIG. 2.

In some embodiments, when the high voltage DC line from each string isreceived at input of a respective inverter, it is subject to maximumpower-point tracking (MPPT) at the string level (e.g., dual MPPT in theexemplary system of FIG. 2). Alternatively, each module, or a number ofindividual modules in the respective strings, may include a DC optimizerthat performs MPPT at the module level or N-module level output, ratherthan at the string level. The various embodiments are compatible witheither centralized or distributed MPPT.

Inverter PCS 206A and 206B may each be coupled to respective energystorage devices 204A and 204B, for storing DC power generated by PVstrings 202A and 202B, or for receiving discharged power from energystorage devices 204A and 204B. It should be appreciated that energystorage devices 204A and 204B in FIG. 2 may be an exemplary commerciallyavailable residential lithium ion battery pack with only its own batteryor with its own battery in addition to a DC/DC buck-boost converter orother topologies. The battery may be a lead acid battery, advanced leadacid battery, flow battery, organic battery, or other battery type. Thevarious embodiments disclosed herein are compatible with numerousdifferent battery chemistries. Various disclosed embodiments will workwith other commercially available energy storage devices as well;however, the embodiments may have particular utility for systems thatuse high voltage energy storage devices (e.g., >48 volts) such as48V-1000V battery packs.

Energy generation system 200 may include power control system 206configured to efficiently manage the power flow between PV strings 202Aand 202B, energy storage devices 204A and 204B, AC grid 208, and loads210. In embodiments, power control system 206 may be configured toinclude more than one inverter PCS, e.g., inverter PCSs 206A and 206B,coupled together in a parallel configuration, where each inverter PCS iscoupled to a respective array of PV strings and an energy storagedevice. An inverter PCS may be different than a conventional inverter inthat inverter PCS may include device components that enable the inverterPCS to communicate and interact with other inverter PCSs, as will bediscussed further herein. Outputs of both inverter PCSs 206A and 206Bmay combine together and be outputted to AC grid 208 or back-up loads210. Having more than one inverter in energy generation system 200allows energy generation system 200 to provide more power to AC grid 208or back-up loads 210. This may be especially useful in situations whereenergy generation system 200 is installed at a location that consumes alot of power, such as a large building (e.g., a resort, villa, or acommercial building).

Having multiple inverter PCSs may result in a more complex energygeneration system. It may be necessary to coordinate the power flow intoand out of power control system 206 in such a way that maximizes thefunctionality, versatility, and return on the investment of energygeneration system 200. Thus, according to embodiments of the presentinvention, one inverter PCS in power control system 206 may bedesignated as the master inverter PCS, while the other inverter PCSs aredesignated as slave inverter PCSs. As an example, inverter PCS 206A maybe designated as the master and inverter PCS 206B may be designated as aslave. In such embodiments, master inverter PCS 206A may be configuredto manage the operation of slave inverter PCS 206B. That way, theoperation of more than one inverter PCSs in power control system 206 maynot conflict with one another, but instead work together as one cohesiveunit to perform a variety of functions.

In embodiments, master inverter PCS 206A may communicate with slaveinverter PCS 206B via communication line 212, which may be a wired orwireless line of communication. For example, communication line 212 maybe a network cable through which signals may be transmitted (eg: rs-485,rs-232, CAN and the like). Alternatively, communication line 212 may bea wireless fidelity (WiFi) connection, Bluetooth connection, radiofrequency (RF) communication, and the like. Communication line 212 mayallow master inverter PCS 206A to send commands to slave inverter PCS206B to control the operation of slave inverter PCS 206B such that itsoperation does not conflict with, but may instead support, the operationof master inverter PCS 206A. In embodiments, slave inverter 206B doesnot have the capability to control the operation of master inverter PCS206A. Instead, slave inverter 206B may only communicate its status ofoperation to master inverter PCS 206A such that master inverter PCS 206Amay better control the operation of power control system 206.Additionally, having only one master inverter PCS 206A provides only onepoint of contact for controlling power control system 206 throughexternal means, thereby simplifying the means through which powercontrol system 206 is controlled. In other embodiments, energygeneration system 200 may not have separate communication line 212. Insuch embodiments, communication may be performed by power linecommunication (PLC) in which communication signals may be transmittedthrough power lines (e.g., power line 209 or 211) that are generallyused for transfer of power.

Although FIG. 2 shows energy generation system 200 as having only twoinverter PCSs 206A and 206B, embodiments are not limited to suchconfigurations. Other embodiments may have more than two inverter PCSs.As an example, a certain embodiment may have three inverter PCSs, or teninverter PCSs in another embodiment or more. It is to be appreciatedthat the number of inverter PCSs may depend on the design requirementsof the energy generation system. Higher output power requirements due tolarger or a greater number of loads may require a larger number ofinverter PCSs. At any rate, regardless of the number of inverter PCSs,one of the inverter PCSs may be a master, and the other inverter PCSsmay be slaves that are managed by the master, as discussed herein.

In order to better understand the operation of power control system 206,it may be necessary to discuss the internal makeup and configuration ofinverter PCSs 206A and 206B, as shown in FIG. 3.

1. Internal Components of an Inverter Power Control System

FIG. 3 illustrates a more detailed block diagram of an exemplary energygeneration system 300 showing additional internal components, overallsystem wiring and inverter wiring compartment interconnections. For easeof discussion, reference to numerical labels without the lettering A orB are directed to the component in general and thus apply to bothcomponents, although it is to be understood that components with thesame numerical indicator but different alphabetical indicators areseparate components.

In embodiments, energy generation system 300 may include power controlsystem 305 that includes more than one inverter PCS 306. Each inverterPCS 306 may be configured to couple to an energy storage device 304 sothat the DC power flowing from the PV strings 302 can be used to deliverDC power to energy storage device 304 for storage. Energy storage device304 has a minimum and maximum associated operating voltage window. Themaximum exposed input voltage limit is, in many cases, lower than thetheoretical maximum DC voltage outputted by PV strings 302 (open circuitvoltage, V_(OC)); thus, various embodiments of the invention include abuck-boost circuit 316 between PV strings 302 and energy storage device304, or between DC/AC inverter 314 and energy storage device 304. Theinclusion of buck-boost circuits 316 or 318 may prevent voltages above asafe threshold from being exposed to energy storage device 304, therebyeliminating the possibility of damage to energy storage device 304 fromovervoltage stress.

As shown in FIG. 3, each inverter PCS 306 has two DC/DC (Buck-Boost)converters 316 and 318. These converters 316 and 318 representalternative embodiments. In the first embodiment, the buck-boost circuitis located between PV string 302 and DC/AC inverter 314 (as depicted byDC/DC buck-boost 316) so that the DC input coming from PV strings 302are always subject to buck or boost, keeping the voltage inputted intoDC/AC inverter 314 at a sufficiently high level for inversion while alsopreventing too high of a voltage from being presented to energy storagedevice 304. In this embodiment, there is no need for a second buck-boostcircuit, e.g., DC/DC buck-boost 318 in inverter PCS 306. In the secondembodiment, the buck-boost circuit is located between DC/AC inverter 314and energy storage device 304 (as depicted by DC/DC buck-boost 318) suchthat the high voltage DC inputs from PV strings 302 or high voltagerectified DC from AC grid side only go through the buck-boost converterwhenever voltage is exposed to energy storage device 304. Eitherembodiment will prevent energy storage device 304 from being exposed toexcessively high voltages generated by PV strings 302. The voltage fromthe PV strings could be as high as 600 Volts, or even 1000 Volts in thecase of a 1 kV PV system in residential applications or 1500 Volts in autility scale PV system.

DC power may be inputted into a DC/AC inverter stage 314 to convert DCpower into AC power. Converted AC power may be outputted to AC grid 308or back-up loads 310. Outputting to either AC grid 308 or back-up loads310 is determined by the configuration of one or more relays disposed ineach inverter PCS 306. For instance, as shown in FIG. 3, each inverterPCS 306 may include anti-islanding (AI) relays 322 and transition relays324 for controlling the destination of outputted AC power. AI relay 322may by a pair of on/off switches that control the flow of power betweeninverter PCS 306 and AC grid 308. If AI relay 322 is closed, then powermay flow from inverter PCS 306 to any of AC grid 308. However, if AIrelay 322 is open, then power may not flow to AC grid 308 but connectthe transfer relay to output to back-up loads 310. AI relay 322 may beany two-pole or multi-pole (depending on the number of AC phases) singlethrow switch or functionally equivalent structure that simultaneouslyopens and closes the power lines in which AI relay 322 is disposed.

In embodiments where AI relay 322 is closed, transition relay 324 maydetermine whether power is operating in an on-grid mode or an off-gridmode. In the on-grid mode, energy generation system 300 is coupled to ACgrid 308 such that power may flow between energy generation system 300and AC grid 308, whereas in the off-grid mode, energy generation systemis disconnected from AC grid 308 and coupled to back-up loads 310 suchthat power may be provided to back-up loads 310 without assistance fromAC grid 308. The position of AI relay 322 and transition relay 324 maydictate which mode of operation energy generation system 300 is in. Forinstance, in a first position, transition relay 324 will operate in theoff-grid mode to allow power to flow from DC/AC inverter stage 314 toon-site back-up loads 310 while DC/AC inverter stage 314 is disconnectedfrom AC grid 308. In a second position, transition relay 324 willoperate in the on-grid mode to allow power to flow to/from AC grid 308while DC/AC inverter stage is disconnected from back-up loads 310.Transition relay 324 may be any suitable mechanical switch or contactoror electrical relay configured to direct power between one input and oneor more outputs. In some embodiments, transition relay 324 may beexternal to the inverter PCS but still be controlled by PCS. Inadditional embodiments, transition relay 324 may be configured to detectthe presence/absence of AC grid 308 and connect the AC output from DC/ACinverter stage 314 to back-up loads 310.

Within the on-grid and off-grid modes, energy generation system 300 mayoperate in more specific modes. In some modes, power may be flowingexclusively from PV strings 302 to energy storage device 304, while inother modes power may be flowing exclusively from PV strings 302 to ACgrid 308 or back-up loads 310. Additionally, in some modes, power may beflowing from energy storage device 304 to AC grid 308 or back-up loads310, while in other modes, power may be flowing from PV strings 302 to acombination of both energy storage device 304 and AC grid 308 or back-uploads 310.

For example, in a first mode, all available DC power from PV strings 302may go to respective energy storage device 304 as a priority, with anysurplus power being supplied to DC/AC inverter stage 314 of inverter PCS306 to be supplied to AC grid 308 or delivered to back-up loads 310. Ina second mode, all generated power may be supplied to DC/AC inverterstage 314 and either used to power back-up loads 310 or supply power toAC grid 308. In yet other modes, energy storage device 304 may bedischarged to DC/AC inverter stage 314 alone and/or with DC power fromPV strings 302 to supply AC power to the AC grid 308 or back-up loads310. Additionally, in a further mode, power may flow from AC grid 308,through DC/AC inverter 314 to charge energy storage device 304, forexample, at a time when PV strings 302 are not generating power anddemand for power is at its lowest point (e.g., after sunset). In variousembodiments, selection of an operating mode may be controlled by logicin inverter PCS 306, or selection could be based on signals from anexternal source. It is to be appreciated that inverter PCS 306 may routethe flow of power between PV strings 302, energy storage device 304, ACgrid 308, and back-up loads 310 as desired.

As can be appreciated herein, each inverter PCS may have a vast numberof operational modes, which can be complex to operate. Adding additionalinverter PCSs in energy generation systems may increase the operationalcomplexity of the energy generation system. Thus, embodiments hereinminimize this complexity by designating one inverter PCS as a master,and the rest of the inverter PCSs as slaves. The master inverter PCS maymanage the operation of slave inverter PCS such that commands arereceived and transmitted from one inverter PCS (e.g., the masterinverter PCS).

As shown in FIG. 3, inverter PCS 306A is designated as the masterinverter PCS, and inverter PCS 306B is designated as the slave inverterPCS. Although FIG. 3 only shows one slave inverter PCS, embodimentshaving more slave inverter PCSs are also envisioned herein. Masterinverter PCS 306A may communicate with slave inverter PCS 306B to enablemaster inverter PCS 306A to send commands to and receive statusinformation from slave inverter PCS 306B. In embodiments, communicationdevice 320A in master inverter PCS 306A communicates with communicationdevice 320B in slave inverter PCS 306B via communication line 312. Eachcommunication device may be a device suitable for sending and/orreceiving communication signals. For instance, each communication devicemay be an antenna or a communication cable receptacle coupled to amicrocontroller, field programmable gate array (FPGA), applicationspecific integrated circuit (ASIC), and the like, configured to interactwith the relevant antenna or receptacle.

In embodiments, master inverter PCS 306A may manage the operations ofslave inverter PCS 306B. For instance, master inverter PCS 306A may senda command to slave inverter PCS 306B that instructs slave inverter PCS306B to operate in any of the operating modes mentioned herein. In oneexample, master inverter PCS 306A may send a command to slave inverterPCS 306B to output AC power to AC grid 308, which results in slaveinverter PCS 306B closing its AI relay switch 322B and configuringtransition relay 324B to direct AC power to AC grid 308. Accordingly,master inverter PCS 306A may instantaneously control when relays 322 and324 in slave inverter PCS 306B change positions. This instantaneouscontrol allows master inverter PCS 306A to operate both inverter PCSs(master and slave) simultaneously such that the power control systemoperates as a single unit. For instance, master inverter PCS 306A andslave inverter PCS 306B may simultaneously transition their relays 322and 324 to output power to AC grid 308. Sometime thereafter, masterinverter PCS 306A and slave inverter PCS 306B may then simultaneouslytransition their relays 322 and 324 to operate in a different mode,e.g., output AC power to back-up loads 310. In some embodiments, ACoutput from both inverter PCSs (master and slave) may be turned off,e.g., transitioning AI relays 322A and 322B into the open position,during service/maintenance for personnel safety.

It is to be appreciated that master inverter PCS 306A may manage theoperation of slave inverter PCS 306B, but this control is not mutual,meaning slave inverter PCS 306B cannot send a command to master inverterPCS 306A to manage the operation of master inverter PCS 306A. By havingonly one master inverter PCS 306A, and one or more slave inverter PCS306B, the communication routes are minimized and the operationalcomplexity is minimized. Commands may originate from one inverter PCS;thus, there is minimal chance that conflicting commands will be sent inthe energy generation system.

The operation of master inverter PCS 306A may be controlled externallyand dynamically, such as by a technician or a customer. In someembodiments, the position of the transition relay may be controlled bythe master inverter PCS 306A in accordance with an algorithm orpredetermined program.

As mentioned above, energy generation system 300 may transition betweenoff-grid and on-grid modes. It is to be appreciated that if thetransition from off-grid to on-grid operation is not performed properly,a sudden influx of AC power from the AC grid or huge power draw frommany back-up loads may damage one or more inverter PCSs. For instance,in conventional energy generation systems, inverters may detect when totransition from off-grid to on-grid mode. The inverters may each beseparately configured to detect when AC power begins to be provided bythe AC grid. Upon detection of the presence of the AC grid, eachinverter may transition from off-grid mode to on-grid mode. The problemwith this is that due to differences in measurement and timing accuracyduring synchronization, each inverter may transition to on-grid mode ata different instance in time. This may cause damage to the firstinverter to switch to on-grid mode because the first inverter to switchto on-grid mode will receive the entire AC power from the AC grid orhuge power draw from many back-up loads (when all inverter PCS arebacking-up all loads combined). Such a large influx of power may be toomuch for one inverter to bear, thereby causing damage to the inverterinternal components and resulting in malfunction of the energygeneration system.

To prevent this from occurring, embodiments herein incorporate a centralAC disconnect 326 positioned along an AC power bus between AC grid 308and inverter PCSs 306A and 306B. Central AC disconnect 326 may stall theinflux of power until all inverter PCSs have been transitioned toon-grid mode to avoid the occurrence of one inverter PCS from bearingthe entire influx of power from AC grid 308 or from back-up loads 310during off-grid mode. The details of this operation will be discussedfurther herein.

2. Central AC Disconnect

In embodiments, a central AC disconnect may be positioned along an ACbus between inverter PCSs and an AC grid. The central AC disconnect maybe configured to open and close the AC bus to control power flow from ACgrid 308 such that damage to inverter PCS caused by an influx of ACpower from the AC grid may be prevented.

FIG. 4 shows an exemplary energy generation system 400 having PV strings402A-402C, energy storage devices 404A-404C, inverter PCSs 406A-406C,and a central AC disconnect 426, according to embodiments of the presentinvention. Inverter PCS 406A is the master, and inverter PCSs 406B and406C are the slaves. Energy generation system 400 is configured toprovide AC power to AC grid 408 and back-up loads 410. As shown in FIG.4, energy generation system 400 includes three PV strings 402A-402Ccoupled to three inverter PCSs 406A-406C independently. Details ofcomponents within each inverter PCS discussed herein with respect toFIG. 3 are not shown in FIG. 4 for ease of discussion.

Outputs of inverter PCSs 406A-406C may combine into an AC bus 414 beforeoutputting to AC grid 408. Central AC disconnect 426 may be positionedalong AC bus 414 to control power flow between inverter PCSs 406A-406Cand AC grid 408. In embodiments, central AC disconnect 426 may includedisconnect switch 428 that is configured to open AC bus 414 to preventcurrent flow. Disconnect switch 428 may be electrically controlled suchthat a signal may control whether disconnect switch 428 is activated(i.e., in the closed position) or not activated (i.e., in the openposition). In certain embodiments, disconnect switch 428 is controllableby master inverter PCS 406A, meaning master inverter PCS 406A maydictate when disconnect switch 428 is activated. In certain embodiments,disconnect switch 428 is a mechanical switch or contactor or electricalrelays.

As shown in FIG. 4, with central AC disconnect switch 428 is in the openposition, thus preventing power flow between inverter PCSs 406A-406C andAC grid 408. In such embodiments, energy generation system 400 may beoff-grid, i.e., operating independently from AC grid 408. Duringoff-grid operation, AC grid 408 may not be providing power to energygeneration system 400. For instance, AC grid 408 may be offline as aresult of a black out. When AC grid 408 is back online, disconnectswitch 428 may close and energy generation system 400 may transitionfrom off-grid to on-grid operation. When energy generation system 400transitions from off-grid to on-grid operation, disconnect switch 428may be activated and power may flow from AC grid 408 to inverter PCSs406A-406B.

To ensure proper transition from off-grid to on-grid operation, centralAC disconnect 426 may include a sensor 430 configured to monitor AC bus414 for determining when AC power is being provided by AC grid 408.Sensor 430 may be communicatively coupled to master inverter PCS or allinverter PCSs, e.g., inverter PCS 406A-406C, such that inverter PCSs406A-406C can monitor signals from sensor 430 to determine when AC poweris being provided by AC grid 408. In embodiments, sensor 430 may be avoltage or current meter that can detect the presence of AC power in ACbus 414. Incorporating central AC disconnect 426 according toembodiments herein may eliminate the possibility of overloading oneinverter PCS when AC power from AC grid 408 becomes available, asdiscussed further in FIGS. 5A and 5B.

FIGS. 5A and 5B illustrate the operation of energy generation system 400during the transition from off-grid to on-grid mode. The bolded arrowsindicate active power and/or communication lines operating around thetime of transition between off-grid to on-grid operation. As shown inFIG. 5A, when power from AC grid 408 is available, sensor 430 may detectAC power in a part of AC bus 414 between disconnect switch 428 and ACgrid 408. Sensor 430 may then send detection signal 432 to inverter PCSs406A-406B indicating that AC power is now available, which then causesinverter PCSs 406A-406C to switch to on-grid mode, e.g., transitionrelays 324 are switched in a position to receive power from AC grid 308as discussed herein with respect to FIG. 3. Once all inverter PCSs havetransitioned to on-grid mode, then central AC disconnect 426 may closedisconnect switch 428 as illustrated in FIG. 5B. With disconnect switch428 closed, AC power may simultaneously flow through inverter PCSs406A-406C. Enabling inverter PCSs 406A-406C to transition to on-gridmode before disconnect switch 428 is closed prevents the situation whereone inverter PCS bears the entire AC power from AC grid 408.Accordingly, damage to an inverter PCS caused by an overload of incomingAC power may be avoided.

In embodiments, each inverter PCS 406A-406C may receive detection signal432 from sensor 430 to determine when to switch to on-grid mode. Inother embodiments, only master inverter PCS 406A may receive detectionsignal 432. In such embodiments, master inverter PCS 406A may, upondetecting availability of AC power from detection signal 432, sendcommands to slave inverter PCSs 406B and 406C via communication lines412 and 413 to cause slave inverter PCSs 406B and 406C to transition toon-grid mode.

B. Charging Operation by Master Inverter PCS

According to embodiments, designating one inverter PCS as a master andthe other inverter PCSs as slaves enables operations of energygeneration systems that would otherwise be substantially morecomplicated to perform without the designation of master and slaveinverter PCSs. This may be particularly true for situations where anenergy generation system has multiple inverters and multiple energystorage devices, but only one inverter is coupled to an array of PVstrings, as shown in FIG. 6. In some embodiments, more than one invertercould be coupled to an array of PV strings but not necessary allinverters.

FIG. 6 illustrates an exemplary energy generation system 600 includingone array of PV strings 602, master inverter PCS 606A, slave inverters660B and 606C, and energy storage devices 604A-604C. Each inverter PCSmay output to either an AC grid 608 or back-up loads 610. As shown inFIG. 6, PV strings 602 may output DC power to only master inverter PCS606A. This energy generation system configuration may be a result ofcost or spatial constraints and power requirements established by acustomer of energy generation system 600. Having only one array of PVstrings saves cost as more arrays of PV strings increases cost.Additionally, having more inverters PCSs allows energy generation system600 to provide more power to drive back-up loads 610, such as for alarge commercial building or a large residential building. Furthermore,having more energy storage devices allows energy generation system 600to store more power so that energy generation system 600 can providepower to drive back-up loads 610 for a longer period of time as extendedenergy capacity when daylight or AC power from AC grid 608 isunavailable.

As can be appreciated by one skilled in the art, operating energygeneration system 600 may be complex. There is only one source of DCpower, yet there are multiple inverters, energy storage devices, andoutput destinations. Configuring the power control system to operate asone cohesive unit to store DC power in the energy storage devices andalso output AC power to AC grid 608 or back-up loads 610 withconventional inverters that do not have master of slave designations canbe extremely complicated. However, according to embodiments herein, thisoperation can be simplified by having one inverter control the operationof all inverters to manage the flow of power between PV strings 602 andAC grid 608 or back-up loads 610.

For instance, master inverter PCS 606A may control the operation ofslave inverter PCSs 606B and 660C. In one embodiment, master inverterPCS 606A may receive DC power from PV strings 602 and evenly distributethe DC power to energy storage devices 604A-604C by commanding slaveinverter PCSs 606B and 606C to store the DC power received from masterinverter PCS 606A into respective energy storage devices 604B and 604C.In another embodiment, as shown by the bolded arrows in FIG. 6, masterinverter PCS 606A may be capable of outputting AC power to back-up loads610 while also storing surplus DC power to energy storage devices 604Band 604C of slave inverter PCSs 606B and 606C, respectively, via ACcoupling. In this embodiment, master inverter PCS 606A may direct someAC power to drive back-up loads 610 while directing some DC power toslave inverter PCSs 606B and 606C for storage in energy storage devices604B and 604C. Master inverter PCS 606A may send commands to slaveinverter PCSs 606B and 606C through communication lines 612 and 613 thatinstruct slave inverter PCSs 606B and 606C to store the received DCpower into energy storage devices 604B and 604C. In other embodiments,any one of slave inverter PCSs 606B or 606C may output AC powerconverted from its respective energy storage device 604B or 604C whileother inverter PCSs store DC power. It is to be understood that anycombination of outputting AC power and storing DC power may be performedunder the command of master inverter PCS without departing from thespirit and scope of the present invention.

As can be appreciated herein, the operation of energy generation system600 is greatly simplified by having one inverter PCS act as the managerof power flow between PV strings 602 and AC grid 608/back-up loads 610.By implementing master and slave inverter PCSs, only one device is incharge of operating the functions of energy generation system 600.Having only one device be in charge of the operations substantiallysimplifies the operation of energy generation system 600 while alsomaximizing the efficiency and functionality of the energy generationsystem.

II. Energy Generation System Configured for Elongated Power Utilization

In addition to an energy generation system configured to have aplurality of inverter PCSs to output a greater magnitude of AC power,embodiments may also include an energy generation system configured tohave a plurality of energy storage devices coupled to a single inverterPCS for providing usable energy for an elongated period of time and/orgreater magnitude of AC power. For example, according to embodiments, anenergy generation system may include at least two energy storage devicescoupled to an inverter PCS. DC power generated by an energy generationdevice, such as an array of PV modules, may be stored in the energystorage devices. The stored energy may be discharged at a later time andconverted into AC power by the inverter PCS. The converted AC power maythen be outputted to an AC grid or back-up loads. According toembodiments, the inverter PCS may be communicatively coupled to eachenergy storage device for operating the energy storage devices.Alternatively, one of the energy storage devices may be designated as amaster, while other energy storage devices may be designated as slaves.The master inverter PCS may mange the operation of slave energy storagedevices. Embodiments herein increase the duration of time in which theenergy generation system may provide power to AC grid or back-up loads.Increasing the duration of time may be particularly useful in situationswhere the installation site does not have reliable access to a utilitygrid, or where sunlight is not consistent or plentiful or where moreenergy storage is required for load shifting application.

FIG. 7 illustrates an exemplary energy generation system 700 having aplurality of energy storage devices (e.g., energy storage devices704A-704C). Each of the energy storage devices 704A-704C may be coupledto inverter PCS 706 such that inverter PCS 706 may draw DC power from,and store DC power in, energy storage devices 704A-704C. Larger numbersof energy storage devices increases the complexity and logistics ofmanaging power flow across the energy generation system. Inefficientpower flow management may result in under-utilization of the energygeneration system and decreased performance, ultimately resulting inmonetary losses to the customer.

According to embodiments of the present invention, the plurality ofenergy storage devices may be part of an energy storage system. Theenergy storage system may include a battery combiner box in addition tothe plurality of energy storage devices. The battery combiner box maycombine DC power being charged/discharged from the energy storagedevices to an inverter PCS interface. The battery combiner box may alsodistribute generated DC power to the plurality of energy storage devicesfor storage. According to embodiments, one energy storage device may bedesignated as a master energy storage device and configured to managethe operations of the other energy storage devices. The energy storagesystem will be discussed in detail further herein with respect to FIG.8.

A. Energy Storage System

FIG. 8A illustrates an exemplary energy generation system 800 having anenergy storage system 801 according to embodiments of the presentinvention. As shown, an array of PV strings 802 is coupled to input ofan inverter PCS 806. Similar to inverter PCS 306A and 306B in FIG. 3,inverter PCS 806 may include DC/DC buck-boost converter 820 (or DC/DCbuck-boost converter 826 in the alternative) and DC/AC inverter stage822. The operations of buck-boost converters 820 and 826 and DC/ACinverter stage 822 are substantially similar to the correspondingcomponents in inverter PCS 306A and 306B in FIG. 3.

Also, similar to array of PV strings 302A and 302B, array of PV strings802 may include a plurality of PV modules (not shown) connected seriallywith an additive direct current (DC) voltage somewhere between 100 and1000 volts or even higher, depending on such factors as the number ofpanels, their efficiency, their output rating, ambient temperature andirradiation on each panel. PV strings 802 may also include a maximumpower-point tracking (MPPT) system for maximizing the power output ofeach array of PV strings. In some embodiments, the MPPT system may be adual MPPT PV system as shown in FIGS. 2 and 3.

Energy generation system 800 may only have a single inverter PCS 806.Thus, the magnitude of power capable of being output by inverter PCS 806may not be as high as what may be achievable by a parallel-coupledplurality of inverter PCSs (such as the embodiment discussed in FIGS. 2and 3). However, energy generation system 800 may output the lowermagnitude of power for a longer period of time due to the large energystorage capacity from more than one energy storage device. In someembodiments, the battery combiner box, DC/DC buck-boost 826, and DC/ACinverter 822 may support more power (aggregated outputs of all theenergy storage devices).

According to embodiments of the present invention, energy storage system801 may include a plurality of energy storage devices 804A-804Cconfigured to charge and discharge DC power. Having plurality of energystorage devices 804A-804C in energy generation system 800 allows energygeneration system 800 to provide AC power to AC grid or back-up loads810 for an elongated period of time. This may be especially useful insituations where the energy generation system 800 is installed at alocation that does not have access to a utility grid or at a locationthat does not receive much sunlight (e.g., a home in a remote locationwithout access to a utility grid, or a home in a tropical region withlots of clouds and rain). This may also be useful in situations whereenergy generation system 800 is installed at a location that does notconsume a lot of power, such as a small building (e.g., a single familyhome, a hut, or a small commercial building).

In embodiments, energy storage system 801 may include a battery combinerbox 803 coupled to energy storage devices 804A-804C. Battery combinerbox 803 may route power between inverter PCS 806 and energy storagedevices 804A-804C. For instance, battery combiner box 803 may receivedischarged power from energy storage devices 804A-804C and combine themtogether into DC power bus 812 as a single DC output to inverter PCS806, or battery combiner box 803 may store power from DC power bus 812to energy storage devices 804A-804C.

Having a plurality of energy storage devices may result in a morecomplex energy generation system. It may be necessary to coordinate thepower flow into and out of the plurality of energy storage devices in away that maximizes the functionality and versatility of the energygeneration system. Thus, according to embodiments of the presentinvention, controller 824 may be coupled to energy devices 804A-804C viacommunication lines 814A-814C, respectively. Controller 824 may be adevice configured to manage the operation of energy storage devices804A-804C, such as an FPGA, microprocessor, ASIC, and the like. As shownin FIG. 8A, controller 824 may directly communicate with each energystorage device 804A-804C in a parallel configuration, which allowscontroller 824 to send unique commands to each energy storage devicewith little to no time delay. Accordingly, controller 824 mayorchestrate the operation of energy storage devices 804A-804C such thatthey work together as one cohesive unit to perform a variety offunctions, as will be discussed further herein with respect to FIG. 12.

In embodiments, communication lines 814A-814C may be wired or wirelesstypes of communication. For example, communication lines 814A-814C maybe network cables through which signals may be transmitted (rs-485,rs-232, CAN and the like). Alternatively, communication lines 814A-814Cmay be wireless fidelity (Wi-Fi) connections, Bluetooth connections,radio frequency (RF) connections, and the like. As shown in FIG. 8,communication lines 814A-814C are wireless communication lines. In otherembodiments, energy generation system 800 may not have communicationlines 814A-814C. In such embodiments, communication may be performed bypower line communication (PLC) in which communication signals may betransmitted through power lines that are generally used for transfer ofpower.

Controller 824 is shown to be communicatively coupled to energy storagedevices 804A-804C in an parallel configuration, meaning that separatecommunication lines 814A-814C may allow controller 824 to directlycommunicate with each energy storage device 804A-804C. However,embodiments are not limited to such configurations. Other configurationsmay have a daisy-chained (i.e., serial) communication connection betweencontroller 824 and energy storage devices 804A-804C, as shown in FIG.8B.

FIG. 8B is a simplified diagram illustrating energy generation system800 where controller 824 is communicatively coupled to energy storagedevices 804A-804C in a daisy-chained or serial configuration. In suchconfigurations, controller 824 may indirectly communicate with one ormore energy storage devices 804A-804C. That is, controller 824 may haveto rely on intermediary energy devices for sending commands to energystorage devices 804A-804C. As an example, in order for controller 824 tosend a command to energy storage device 804C, controller 824 may firstsend the command to energy storage device 824A, which then relays theinformation to energy storage device 804B, which then finally getsrelayed to energy storage device 804C. It is to be appreciated that anyconfiguration may be utilized for establishing communication betweencontroller 824 and energy storage devices 804A-804C without departingfrom the spirit and scope of the present invention.

Although FIGS. 8A and 8B illustrate energy generation system 800 ashaving only three energy storage devices 804A-804C, embodiments are notlimited to such configurations. Other embodiments may have more thanthree energy storage devices. As an example, a certain embodiment mayhave four energy storage devices, ten energy storage devices in anotherembodiment, or even more in other embodiments. It is to be appreciatedthat the number of energy storage devices may depend on the designrequirements of the energy generation system. Larger storage capacityrequirements due to longer periods of no sunlight may require a largernumber of energy storage devices.

In order to better understand the operation of energy generation system800, it may be necessary to discuss the internal makeup andconfiguration of battery combiner box 803 and energy storage devices804A-804C in energy storage system 801, as shown in FIG. 9.

B. Components of an Energy Storage System

FIG. 9 is a simplified diagram illustrating the internal components of abattery combiner box 803 and energy storage devices 804A-804C shown inFIGS. 8A and 8B herein, according to an embodiment of the presentinvention. For ease of discussion, reference to numerical labels withoutthe lettering A, B, or C are directed to the component in general andthus apply to all duplicative components, although it is to beunderstood that components with the same numerical indicator butdifferent alphabetical indicators are physically separate components.

As shown, energy storage devices 804A-804C may each include an energystorage component, such as battery 908, fuel cell, or the like, as wellas other components aside from mere energy storage that enable energystorage devices 804A-804C to perform additional functions. As anexample, energy storage devices 804A-804C may each include acommunication component 902, DC/DC converter 904 and a batterymanagement system (BMS) 906, with all of them interconnected, accordingto embodiments of the present invention.

Communication component 902 may be a component for establishingcommunication with controller 824 of inverter PCS 806, as shown in FIG.8. For instance, communication component 902 may be an FPGA,microprocessor, ASIC, and the like, configured to establish a wireless(e.g., WiFi, Bluetooth, or RF) communication with controller 824. BMS906 may be a component that manages the operation of battery 908. Inembodiments, BMS 906 may prevent battery 908 from operating outside ofits operating limits, such as preventing battery 908 from beingovercharged or monitoring the state of the battery, or any other relatedfunctions (charging, discharging, fault protection and the like).

DC/DC converter 904 may be a boost converter, buck converter, or abuck-boost converter configured to alter the output voltage of battery908. DC/DC converter 904 may be configured to standardize the output DCvoltage of battery 908 such that a standard amount of power may beprovided by energy storage device 804 regardless of the specification ofbattery 908. Implementing DC/DC converter 904 is important because itallows flexibility in the type of battery utilized by energy storagesystem 801. Different manufacturers may produce batteries that outputdifferent magnitudes of DC voltages. By incorporating DC/DC converter904 in the energy storage device, the output DC voltage outputted byenergy storage devices 804 may be standardized across all energy storagedevices 804.

In embodiments, batteries 908 and DC/DC converters 904 may includeprotection circuits (not shown) configured to protect batteries 908 andDC/DC converters 904 from operational harm (e.g., harm caused by overvoltages, reverse polarity, over currents, ground faults, surges and thelike). As an example, batteries 908 may include protection circuits forpreventing batteries 908 from short-circuiting when coupled to DC/DCconverters 904. DC/DC converters 904 may include protection circuits forpreventing damage to energy storage devices 804 when an energy storagedevice is connected in series with other energy storage devices.

The output DC voltage of each energy storage device 804 may be routedthrough battery combiner box 803 before being inputted into inverter PCS806. In embodiments, battery combiner box 803 may include one or moredisconnection and protection components 910 configured to sever atransmission of power between an energy storage device and inverter PCS806. For instance, disconnection and protection components 910 may be aswitch, contactor, fuse, relay, circuit breaker, and the like.Disconnection and protection components 910 may protect energy storagedevices 804 from short circuiting, drawing too much current duringoperation of energy generation system 800, or from being connected inthe wrong orientation. In embodiments, disconnection and protectioncomponents 910 are disposed along a power line for routing power betweenenergy storage devices 804 and inverter PCS 806.

In addition to disconnection and protection components 910, batterycombiner box 803 may also include one or more visual indicators 912. Inembodiments, visual indicators 912 may be configured to visuallyindicate whether power, voltage, or current is flowing through a powerline. For example, visual indicators 912 may be light emitting diodes(LEDs) that emit light when power is flowing through a power line alongwhich the visual indicator is disposed, and that do not emit light whenpower is not flowing through the power lines. Visual indicators 912 maybe formed of any other suitable component that does not depart from thespirit and scope of the present invention. In some embodiments,additional visual indicators (not shown) may be disposed in each energystorage device 804 for indicating whether energy storage device 804 isoperational.

Although FIG. 8 illustrates battery combiner box 803 as having threedisconnection and protection components 910A-910C and three visualindicators 912A-912C, it is to be appreciated that embodiments are notlimited to such configurations. The number of disconnection andprotection components 910 and visual indicators 912 may be dependent onthe number of energy storage devices 804. Thus, the number ofdisconnection and protection components 910 and visual indicators 912may be determined according to system design.

DC power outputted by energy storage devices 804 may be combined inbattery combiner box 803 into DC power bus 912 and then outputted toinverter PCS 806. Thus, power flowing through DC power bus 912 may be asum of the individual outputted power from energy storage devices804A-804C.

According to embodiments, the amount of power outputted to DC power bus912 may be determined based upon commands sent from inverter PCS 806.That is, inverter PCS 806 may manage energy storage devices 804 tooutput DC power by sending commands through communication lines814A-814C. Being capable of communicating with energy storage devices804A-804C enables several different charging/discharging schemes forenergy storage devices 804A-804C, as shown in FIG. 10.

C. Operating an Energy Storage System

FIG. 10 illustrates a series of charts corresponding to different waysof operating energy storage devices 804A-804C as enabled by embodimentsof the present invention. Specifically, FIG. 10 illustrates threedifferent schemes for discharging energy storage devices 804A-804C:scheme 1, scheme 2, and scheme 3. Each scheme is represented by two setsof three charts aligned in a row. The three charts represent threeenergy storage devices, such as energy storage devices (ESD) 804A-804C.Each chart of the first set of three charts has an X axis representingtime from T₀ to T_(N), and each chart of the second set of three chartshas an X axis representing time from T_(N) to T_(M), where N and M areintegers and where M is an instance of time after N. The Y axis of allsix charts for each scheme represent an output DC power of thecorresponding energy storage device. Each of the illustrated schemes maybe managed by commands sent from an inverter PCS, such as inverter PCS806 in FIGS. 7, 8A-8B, and 9. For ease of description, schemes 1-3output a total of 6,000 watts (W) to inverter PCS 806 from energystorage system 801. It is to be appreciated that energy storage system801 can output many other magnitudes of wattage, and that 6,000 W isjust one example output voltage for discussion purposes.

In embodiments, scheme 1 pertains to a charging/discharging scheme whereonly one of the energy storage devices is charged/discharged at a timeand in a rotational order. For instance, as shown in FIG. 10, from T₀ toT_(N), energy storage device 804A may output 6,000 W while energystorage devices 804B and 804C are outputting 0 W. Thereafter, from T_(N)to T_(M), energy storage device 804A may stop outputting 6,000 W andoutput 0 W, while energy storage device 804B outputs 6,000 W. Eachenergy storage device may take turns charging/discharging the requiredamount of power. Taking turns distributes cycling stress amongst theenergy storage devices such that one energy storage device is notsubjected to more cycling stress than the other energy storage devices,thereby increasing reliability and life of the energy storage devicesduring continuous operation of the energy storage system.

Scheme 2 pertains to a charging/discharging scheme where all of theenergy storage devices are simultaneously charged/discharged andcombined to output a desired output power. As an example, from T₀ toT_(N), energy storage devices 804A-804C are each outputting 2,000 W,which may be combined to output 6,000 W to an inverter PCS. Thereafter,from T_(N) to T_(M), energy storage devices 804A-804C may continue tooutput 2,000 W to provide a combined 6,000 W to the inverter PCS.Outputting at a lower power reduces stress subjected to the energystorage devices, and it also charges/discharges the stored energy at aslower rate so that the energy storage devices may provide power for anelongated period of time. Thus, all of the energy storage devices maycontinually output 2,000 W from T₀ to T_(M). In embodiments, havingcommunication lines organized in a parallel configuration as shown inFIG. 8A enables scheme 2. By allowing controller 824 to directlycommunicate with energy storage devices 804A-804C, energy storagedevices 804A-804C may simultaneously begin charging/discharging 2,000 Wat the same time without effects from time delays experienced whencommunication lines are organized in serial configurations.

Scheme 3 pertains to a charging/discharging scheme where only one of theenergy storage devices is discharged at a time and in a rotationalorder, but the transition between charging/discharging energy storagedevices is gradual. For instance, as shown in FIG. 10, energy storagedevice 804A may output 6,000 W while energy storage devices 804B and804C are outputting 0 W. At an instance of time between T₀ to T_(M),energy storage device 804A may begin to gradually decrease its outputtedpower while energy storage device 804B begins to gradually increase itsoutputted power. The rate at which energy storage device 804A decreasesand energy storage device 804B increases may be the same such that thenet power output is constant at 6,000 W. This gradual change may repeatas each energy storage device takes turns outputting power. By graduallytransitioning between energy storage devices, the power outputted byenergy storage devices 804A-804C may be uninterrupted, while ensuringthat cycling stress is evenly distributed amongst energy storage devices804A-804C.

As aforementioned herein, the inverter PCS may control the operation ofenergy storage devices according to any of schemes 1-3. Whileembodiments herein enable such complex operations, in some situations, acustomer may not want nor afford all of the capabilities of the energygeneration system aforementioned herein. In this case, the energygeneration system may be simplified to decrease complexity and cost. Forexample, an energy storage system may be configured such that only oneenergy storage device may provide power to inverter PCS at one time, asshown in FIG. 11.

D. Simplified Configurations of an Energy Storage System

FIG. 11 illustrates one embodiment of the present invention where energystorage system 801 is configured to output power to inverter PCS 806from only one energy storage device at a time. As shown, batterycombiner box 803 may include a multi-position transfer switch 918 forselecting between energy storage devices 804A-804C. Switch 918 may be atwo-pole N position transfer switch configured to select between Mavailable energy storage devices, where N and M are integers and N isgreater than or equal to M. Switch 918 may be a two-pole switch forpositive and negative terminals of the power lines. In the example shownin FIG. 10, switch 918 may be a two-pole three position transfer switchconfigured to select between three energy storage devices 804A-804C.

In addition to switch 918, battery combiner box 803 may includedisconnection and connection component 920. Disconnection and connectioncomponent 920 may be similar to disconnection and protection components910 in FIG. 9 in that they may be configured to selectively preventtransmission of power between an energy storage device and inverter PCS806. As shown in FIG. 11, disconnection and connection component 920 maybe disposed along a power lines between switch 918 and inverter PCS 806.According to the configuration of energy storage system 1101, batterycombiner box 1103 may be simpler and cheaper in construction thanbattery combiner box 803 shown in FIG. 9.

According to an embodiment, an energy generation system may besimplified even more to further reduce complexity and cost. Exemplaryenergy generation systems with minimal complexity and cost are shown inFIGS. 12 and 13.

FIG. 12 is a simplified diagram illustrating energy generation system1200 according to an embodiment of the present invention. As shown,energy generation system 1200 may include PV strings 802 coupled to aninverter PCS 806, which is configured to output AC power to an AC grid808 or back-up loads 810. Details of such components may be referencedin the discussion herein with respect to FIG. 8.

In addition to the components mentioned above, energy generation system1200 may also include an energy storage system 1201 containing aplurality of energy storage devices 1202, 1204, and 1206. To minimizecost and complexity of energy storage system 1201, only one of energystorage devices 1202, 1204, and 1206 may be configured to communicatewith inverter PCS 806 while the other energy storage devices are merelyused to store energy for providing added output power. As an example,energy storage device 1202 may be designated as a master configured tocommunicate with inverter PCS 806 and output voltage supplied by its ownbattery 1214 as well as batteries 1216 and 1218 of slave energy storagedevices 1204 and 1206, respectively. Master energy storage device 1202may include a communication device 1208, a battery management system1212, and a DC/DC converter 1210, while slave energy storage devices1204 and 1206 include less components than master energy storage device1202. For instance, slave energy storage devices 1204 and 1206 maymerely include an energy storage component (i.e., battery 1216 and 1218,or battery 1216 and BMS 1212). Communication device 1208, batterymanagement system 1212, and DC/DC converter 1210 in master energystorage device 1202 may be similar to corresponding components in energystorage devices 804 discussed herein with respect to FIG. 9.

As shown in FIG. 12, inverter PCS 806 may communicate with master energystorage device 1202 via communication line 1226, and may charge anddischarge power from master energy storage device 1202 via power lines1220. To supply additional power for outputting to inverter PCS 806, oneor more power lines may be configured to transfer power from slaveenergy storage devices 1204 and 1206 to master energy storage device1202. In one embodiment, master energy storage device 1202 may beserially coupled to slave energy storage devices 1204 and 1206 by powerlines 1222 and 1224 as shown in FIG. 12. In such configurations, powerfrom slave energy storage device 1206 may first flow to slave energystorage device 1204, which may combine its output power to the powerreceived from slave energy storage device 1206 and output the combinedpower to master energy storage device 1202 via power lines 1222. Masterenergy storage device 1202 may then combine its outputted power with thereceived combined power from slave energy storage devices 1204 and 1206to output to inverter PCS 806. DC/DC converter 1210 in master energystorage device 1202 may regulate the outputted combined power accordingto commands sent from inverter PCS 806.

In another embodiment, master energy storage device 1202 may be coupledto slave energy storage devices 1204 and 1206 by power lines 1230 and1232 in a parallel configuration as shown in FIG. 13. In such aconfiguration, power from slave energy storage devices 1204 and 1206 maydirectly flow to master energy storage device 1202 without having toflow through intermediary energy storage devices as required in a serialconfiguration. The received power from slave energy storage devices 1204and 1206 may be all combined by master energy storage device 1202 andthen outputted to inverter PCS 806. DC/DC converter 1210 in masterenergy storage device 1202 may regulate the outputted combined poweraccording to commands sent from inverter PCS 806.

By having only one master energy storage device 1202 for managing theoutput of power from energy storage system 1201, controlling theoperation of energy storage system 1201 may be simplified as inverterPCS 806 only needs to communicate with master energy storage device1202. Furthermore, by omitting components, such as communicationdevices, DC/DC converters, and/or battery management systems in slaveenergy storage devices 1204 and 1206, the cost of each slave energystorage device 1204 and 1206 is reduced, and ideally minimized, whichreduces the total cost of energy generation system 1200. Thus, it is tobe appreciated that even through slave energy storage devices 1204 and1206 may not be able to communicate with inverter PCS 806 or dynamicallyalter its output voltage according to various charging/dischargingschemes as discussed herein with respect to FIG. 10, energy storagesystem 1201 is simpler to operate and substantially cheaper tomanufacture. This may be particularly advantageous for customers whocannot afford, or who have no interest in, the functionalities providedby the more functional energy storage systems discussed herein. It is tobe noted that a detailed discussion of energy storage devices mentionedherein may be referenced in U.S. patent application Ser. No. 14/931,648,filed Nov. 3, 2015, which is herein incorporated by reference in itsentirety for all purposes.

III. Energy Generation System Configured for Multi-Phase Operation

As can be appreciated by the disclosures above, the aforementionedenergy generation systems provide AC power to an AC grid or back-uploads that operate in a single phase. Most common household installationlocations in North America have back-up loads that operate onsingle-phase power; however, installation locations other than commonhousehold locations in North America, such as commercial buildings,large apartment complexes, or other locations in different countries mayhave back-up loads that operate on multi-phase AC power (i.e.,three-phase power 208V or 480V). Having back-up loads that operate onmulti-phase AC power may require different configurations of energygeneration systems, as will be discussed further herein with respect toFIGS. 14, 15, and 16.

A. Waveforms of a Multi-Phase System

To fully understand the differences between multi-phase and single-phaseenergy generation systems, it may be worthwhile to discuss thefundamental differences between voltage waveforms for a single-phase andmulti-phase system. FIGS. 14A and 14B illustrate two charts: chart 1400and 1401. Chart 1400 in FIG. 14A shows a waveform 1402 representing oneperiod of AC voltage for a single-phase system with time increasing tothe right. Single-phase systems operate with only a single wave 1402 ofAC power for every unit of time. In the case of 60 cycle (Hz) powercommon in North America, the period is 1/60 of a second.

Alternatively, chart 1401 in FIG. 14B shows waveforms 1404, 1406, and1408 representing one electrical period of AC voltages for a multi-phasesystem with time increasing to the right. Specifically, chart 1401 isfor a balanced three-phase system where waveforms 1404, 1406, and 1408represent transmission of AC power for each phase of the three-phasesystem. Three-phase is more efficient and commonly used for high voltagepower transmission. Each waveform has the same frequency and voltageamplitude, but the phase at which each waveform propagates is offset bya fraction of one period, which is defined by the reciprocal of thefrequency at which each waveform propagates. For three-phase systems,each waveform may be offset by an equal degree of shift, such as a thirdof the period, e.g., 120° as shown in FIG. 14B.

Multi-phase systems may be advantageous over single-phase systems inthat power transfer to loads may be constant, which helps reducegenerator and motor vibrations. Additionally, multi-phase systems canproduce a rotating magnetic field with a specified direction andconstant magnitude, which simplifies the design of electric motors, suchas those use in some commercial and residential appliances. Furthermore,in multi-phase system, if one phase fails, the other phases may continueto provide power. The circuits configured to operate with multi-phasesystems, particularly three-phase systems, are shown in FIGS. 15A and15B.

B. Circuits of a Multi-Phase System

FIGS. 15A and 15B illustrate exemplary circuit configurations forthree-phase systems. One configuration is a “Wye” configuration shown inFIG. 15A which includes three AC current sources 1502, 1504, and 1506,each generating an AC waveform that is offset at a phase from the otherAC waveforms, as discussed herein with respect to FIG. 14B. AC currentsources 1502, 1504, and 1506 may provide AC power to three separatelines: line 1, line 2, and line 3, for driving loads 1508, 1510, and1512. In the Wye configuration, each AC current source 1502, 1504, and1506 may be a ground-referenced voltage that shares an optional commonneutral node 1508. Each AC current source 1502, 1504, and 1506 mayindependently output power to a load. For instance, as shown in FIG.15A, loads 1508, 1510, and 1512 may be separate loads that are driven byseparate AC current sources. Thus, if one AC current source were tofail, the other loads may be driven by the other AC current sources thatare still operable.

Another configuration is a “Delta” configuration shown in FIG. 15B whichincludes three AC current sources 1522, 1524, and 1526, each generatingan AC waveform offset in phase from one another. AC current sources1522, 1524, and 1526 may provide AC power to line 1, line 2, and line 3,for driving loads 1528, 1530, and 1532. In the Delta configuration, eachAC current source 1502, 1504, and 1506 may be connected in series toform a closed circuit.

As can be appreciated by the Wye and Delta configurations of FIGS.15A-B, multi-phase operations can be performed by three separatesingle-phase systems where each single-phase system produces AC powerthat is offset from one another by a phase. This configuration may beimplemented in an energy generation system, as discussed with respect toFIG. 16 herein.

C. Energy Generation System for Multi-Phase Systems

FIG. 16 is a simplified block diagram illustrating energy generationsystem 1600 for providing AC power to a multi-phase AC grid ormulti-phase back-up loads, according to embodiments of the presentinvention. Energy generation system 1600 may include three subsystems:subsystem A, subsystem B, and subsystem C, where each subsystem isconfigured to provide AC power in each phase of the multiple phases.When combined, the three subsystems A, B, and C form a multi-phasesystem for providing multi-phase power. Multi-phase energy generationsystem 1600 may be particularly useful for large buildings that spanacross a large area or demand high amounts of power, or for buildingsthat are located in regions whose laws require three-phase power systemsor that house equipment requiring three-phase power. There may becommunication between the three subsystems to coordinate for properphase balance under three-phase AC grid or back-up loads.

According to embodiments of the present invention, each subsystem may bea single-phase energy generation system having an inverter PCSconfigured to output single-phase AC power converted from DC powergenerated from arrays of PV strings or discharged by an energy storagedevice. For instance, inverter PCS 1606A may be configured to receive DCvoltage at an input of the inverter PCS 1606A and may store the DCenergy in energy storage device 1604A or convert the received DC energyto single-phase AC power and output the converted single-phase AC powerto AC grid 1608A or back-up loads 1610A, each operating at acorresponding single phase. Inverter PCSs 1606A-1060C may be similar inoperation and construction to inverter PCSs 306A and 306B discussedherein with respect to FIG. 3.

Outputted AC power from respective inverter PCSs in subsystems A-C maybe outputted to AC grid 1608A-1608C or back-up loads 1610A-1610C,respectively. Each respective AC grid may operate in a phasecorresponding to a respective subsystem. As an example, single-phaseinverter PCS 1606A in subsystem A may be configured to output AC powerin phase 1, single-phase inverter PCS 1606B in subsystem B may beconfigured to output AC power in phase 2, and single-phase inverter PCS1606C in subsystem C may be configured to output AC power in phase 3.The phases 1-3 may each be outputted to respective AC grids 1608 orback-up loads 1610. Thus, each inverter PCS 1606A-1606C in multi-phaseenergy generation system 1600 may correspond to the AC sources discussedherein with respect to FIGS. 15A and 15B. In embodiments, back-up loadsfor each subsystem may be different loads in an installation site. As anexample, back-up loads 1610A may be appliances in a kitchen, back-uploads 1610B may be devices in a bedroom, and back-up loads 1610B may belighting at the installation site.

Phases of output AC power from subsystems A-C may be offset from oneanother, as discussed herein with respect to FIG. 14B. Thus, inverterPCSs may need to be coordinated with one another such that no twoinverter PCSs are outputting at the same phase. To coordinate the phasesof AC outputs, one inverter PCS may be designated as a master while theother inverter PCSs are designated as slaves. As shown in FIG. 16,inverter PCS 1606A may be designated as a master, and inverter PCSs1606B and 1606C may be designated as slaves. Master inverter PCS may beconfigured to manage the operations of slave inverter PCSs 1606B and1606C by sending commands to and receiving status information from slaveinverter PCSs 1606B and 1606C through communication lines 1612 and 1613.For instance, master inverter PCS 1606A may output AC power in phase 1,and may send a command to slave inverter PCS 1606B to output AC power inphase 2, and a command to slave inverter PCS 1606C to output AC power inphase 3. Thus, outputted AC power from subsystems A-C may form amulti-phase system including phases 1-3.

Multi-phase energy generation systems are more robust and flexible thansingle-phase energy generation systems. For example, operating an energygeneration system configured for multi-phase operation may be capable ofoutputting AC power even when one of the subsystems fails. For example,array of PV strings 1602B and/or energy storage device 1604B ofsubsystem B may fail during operation of energy generation system 1600.In this case, subsystem B may not be capable of outputting AC power toAC grid 1608B or back-up loads 1610B. While subsystem B may not be ableto output AC power in phase 2, subsystems A and C may still be capableof outputting AC power in phases 1 and 3. Additionally, operating anenergy generation system configured for multi-phase operation may becapable of outputting different magnitudes of AC power to different ACgrids or on-site back-up loads. Outputting different magnitudes of ACpower may be suitable for instances where voltage imbalance is detectedin the multi-phase system. The different magnitudes of outputted voltagemay compensate for the voltage imbalance (i.e., re-balancing themulti-phase system), thereby minimizing damage to one or more loads.

Although FIG. 16 illustrates communication lines 1612 and 1613 arearranged in a serial configuration, embodiments of the invention neednot be so limited. Other embodiments may have communication lines 1612and 1613 arranged in a parallel configuration, or any other suitableconfiguration suitable for allowing master single-phase inverter PCS1606A to communicate with slave single-phase inverter PCSs 1606B and1606C.

What is claimed is:
 1. A power control system, comprising: a firstinverter power control system; and a second inverter power controlsystem coupled in a parallel configuration with the first inverter powercontrol system, wherein both first and second inverter power controlsystems each comprise: an input configured to receive direct current(DC) power; a DC to alternating current (AC) inverter stage configuredto receive the DC power input; an anti-islanding relay coupled to theoutput of the DC/AC inverter stage; and a transition relay coupled tothe anti-islanding relay, the transition relay comprising a firstterminal coupled to the anti-islanding relay, a second terminal coupledto an AC grid, and a third terminal coupled to one or more onsiteback-up loads, wherein the transition relay is configured to route anoutput of the inverter power control system between the one or moreonsite back-up loads and the AC grid, wherein the first inverter powercontrol system is designated as a master that is configured to controlthe operation of the second inverter power control system designated asa slave.
 2. The power control system of claim 1, wherein the firstinverter power control system is configured to control the transitionrelay of the second inverter power control system.
 3. The power controlsystem of claim 1, wherein both of the first and second inverter powercontrol systems further comprise a DC to DC converter stage configuredto receive and step up or step down the DC power input to a levelsuitable for inversion.
 4. The power control system of claim 1, furthercomprising a communication line coupled between the first and secondinverter power control systems.
 5. The power control system of claim 4,wherein the first and second inverter power control systems comprise afirst and second communication device, respectively, the communicationline coupling the first communication device to the second communicationdevice.
 6. The power control system of claim 4, wherein thecommunication line is a wireless communication line.
 7. An energygeneration system, comprising: one or more photovoltaic (PV) strings; apower control system comprising a plurality of inverter power controlsystems connected in a parallel configuration and coupled to the one ormore PV strings, each inverter power control system comprising: an inputconfigured to receive direct current (DC) power; a DC to alternatingcurrent (AC) inverter stage configured to receive the DC power input; ananti-islanding relay coupled to the output of the DC/AC inverter stage;a transition relay coupled to the anti-islanding relay, the transitionrelay comprising a first terminal coupled to the anti-islanding relay, asecond terminal coupled to an AC grid, and a third terminal coupled toone or more onsite back-up loads, wherein the transition relay isconfigured to route an output of the inverter power control systembetween the one or more onsite back-up loads and the AC grid, whereinone of the plurality of inverter power control systems is designated asa master that is configured to control an operation of another inverterpower control system designated as a slave; and one or more energystorage devices coupled to the plurality of inverter power controlsystems.
 8. The energy generation system of claim 7, further comprisinga central AC disconnect coupled between the plurality of inverter powercontrol systems and the AC grid.
 9. The energy generation system ofclaim 8, wherein the central AC disconnect is configured tosimultaneously connect and disconnect the plurality of inverter powercontrol systems to the AC grid.
 10. The energy generation system ofclaim 8, further comprising a communication line coupling the central ACdisconnect with the plurality of inverter power control systems.
 11. Thepower control system of claim 7, wherein one inverter power controlsystem is configured to control the other inverter power control systemsof the plurality of inverter power control systems.
 12. The powercontrol system of claim 7, wherein each inverter power control systemfurther comprises a DC to DC converter stage configured to receive andstep up or step down the DC power input to a level suitable forinversion.
 13. The power control system of claim 7, further comprisingcommunication lines coupled between the plurality of inverter powercontrol systems.
 14. The power control system of claim 13, wherein eachinverter power control system of the plurality of inverter power controlsystems comprises a communication device, the communication linescoupling together the communication devices in the plurality of inverterpower control systems.
 15. The power control system of claim 13, whereinthe communication lines are wireless communication lines.
 16. A method,comprising: receiving direct current (DC) power at a first inverterpower control system and at a second inverter power control system;generating a command at the first inverter power control system; sendingthe command from the first inverter power control system to the secondinverter power control system; receiving, at the second inverter powercontrol system, the command from the first inverter power controlsystem, the command instructing the second inverter power control systemto output power to at least one of an AC grid or one or more onsiteback-up loads by switching a transition relay coupled to ananti-islanding relay, the transition relay comprising a first terminalcoupled to the anti-islanding relay, a second terminal coupled to the ACgrid, and a third terminal coupled to the one or more onsite back-uploads.
 17. The method of claim 16, wherein at least a portion of thereceived DC power at the second inverter power control system isoutputted to an energy storage device.
 18. The method of claim 16,wherein the command instructs the second inverter power control systemto alter a position of a transition relay in the second inverter powercontrol system.
 19. The method of claim 18, wherein the position of thetransition relay is altered to output power to an AC grid.
 20. Themethod of claim 18, wherein the position of the transition relay isaltered to output power to on-site back-up loads.